It is well established in the semiconductor industry that production of smaller features on integrated circuits is highly desirable. Market trends in integrated-circuit based electronic equipment are towards smaller, faster, lighter equipment with ever greater levels of function. In an effort to build faster and more powerful computer hardware, engineers are steadily seeking to shrink the size of circuit elements and data storage devices. With conventional optical lithographic techniques, there are some inherent limits on how narrow (or fine) a line (or feature) can be laid out and fabricated in a semiconductor device. A similar problem is evident with regard to inspecting semiconductor circuit elements for flaws, which demands a resolution generally at least 10 times (one order of magnitude) finer than the smallest element.
Photolithography is a common technique employed in the manufacture of semiconductor devices. Typically, a semiconductor wafer is coated with a layer of light sensitive resist material (photoresist). Using a patterned mask or reticle, the wafer is exposed to projected light from an illumination source, typically actinic light, which manifests a photochemical effect on the photoresist, which is ultimately (typically) chemically etched away, leaving a pattern of photoresist "lines" on the wafer corresponding to the pattern on the mask or reticle. The patterned photoresist on the wafer is also referred to as a mask, and the pattern in the photoresist mask replicates the pattern on the image mask (or reticle).
In current photolithographic apparatus, light having at least a substantial visible content is typically employed. Visible light has a frequency on the order of 10.sup.15 Hz (Hertz), and a wavelength on the order of 10.sup.-6 -10.sup.-7 meters. The following terms are well established: 1 .mu.m (micrometer) is 10.sup.-6 meters; 1 nm (nanometer) is 10.sup.-9 meters; and 1 .ANG. (Angstrom) is 10.sup.-10 meters.
Among the problems encountered in photolithography are nonuniformity of source illumination and point-to-point reflectivity variations of photoresist films. Both of these features of current photolithography impose undesirable constraints on further miniaturization of integrated circuits. Small and uniformly sized features are, quite evidently, the object of prolonged endeavor in the field of integrated circuit design. Generally, smaller is faster, and the smaller the features that can be reliably fabricated, the more complex the integrated circuit can be.
With regard to uniformity of source illumination, attention is directed to commonly-owned U.S. Pat. No. 5,055,871, issued to Pasch. As noted in that patent, non-uniformities in the illuminating source will result in non-uniformities of critical dimensions (cd) of features (e.g., lines) formed on the semiconductor device, and the illumination uniformity if photolithographic apparatus will often set a limit to how small a feature can be formed. There usually being a small "error budget" associated with any integrated circuit design, even small variations in illumination intensity can be anathema to the design goals.
With regard to reflectivity of photoresist films, it has been observed that minor thickness variations in a photoresist film will cause pronounced local variations in how efficiently the illuminating light is absorbed (actinically) by the photoresist film, which consequently can adversely affect the uniformity of critical dimensions (cd) of features (such as polysilicon lines or gates) sought to be formed in a layer underlying the photoresist. This problem is addressed in commonly-owned, copending U.S. patent application No. 07/906,902, filed Jun. 29, 1992 by Michael D. Rostoker, which discusses techniques for applying a substantially uniform thickness layer of photoresist, and which is incorporated by reference herein.
Another, more serious problem with photolithography is one of its inherent resolution. The cd's of the smallest features of today's densest integrated circuits are already at sub-micron level (a "micron" or ".mu.m" is one millionth of a meter). Such features are only slightly larger than a single wavelength of visible light, severely pushing the limits of the ability of visible light techniques to resolve those features. As integrated circuit features become smaller, the demand for more nearly "perfect" optical components increases. At some point, however, such optics become impractical and inordinately expensive, or even impossible to produce. Although the resolving power of light, vis-a-vis submicron semiconductor features is being stretched to its limit, the ability to etch (wet, dry, chemical, plasma) features on a semiconductor wafer is not limited by wavelength.
As the limits of conventional focusing optics have been pushed out of the realm of usefulness for photo-lithography, techniques such as e-beam (electron-beam) lithography have surfaced. Focusing an electron beam requires a different type of "optics", generally involving electromagnetic and/or electrostatic fields to effect focusing and deflection of the beam. Further, the nature of e-beam lithography is such that it can only be carried out in a vacuum.
In a general sense, dealing with objects smaller than the wavelengths of visible light is becoming more and more common in contemporary science and technology. For example, with regard to the issue of inspectability, there has appeared a family of new microscopes capable of mapping atomic and molecular shapes, electrical, magnetic and mechanical properties, and temperature variations at a higher resolution than ever before, without the need to modify the specimen or expose it to damaging, high-energy radiation. These microscopes are known as scanned-probe microscopes, and are typified by the scanning-tunneling microscope (STM). The STM is relatively small and inexpensive, compared to other high-resolution microscopes, but provides unprecedented resolution and accuracy. For example, the topology of a surface can be detected, down to the atomic level in some instances. In an extension of the STM technology, STM equipment has been demonstrated to be capable of manipulating (picking and placing) individual atoms on a surface of a substrate.
High-resolution imaging, however, presents an altogether different problem. Short-wavelength lithographic techniques, such as ultraviolet and X-ray lithography, have been proposed and/or are in use. These techniques overcome the visible light resolution barrier, but exhibit difficulties of their own, not the least of which are expense, difficulty to control, cumbersome processing environments, unpredictable radiation source characteristics (i.e., poor source "fluency"), etc..
As used herein, the term "lithography" refers to the process whereby a pattern of lines and the like is formed within a layer of a material (e.g., photoresist) on a semiconductor device. The pattern, which represents `converted` material surrounded by `unconverted` material (or vice-versa) is used, in subsequent processing steps, to form corresponding structures in an underlying layer (e.g., polysilicon) on the semiconductor device. As mentioned above, techniques such as ultraviolet and X-ray lithography hold promise for forming finer features (i.e., finer patterns, hence smaller structures) in semiconductor devices. Given the object of forming ever finer features, what is needed are even higher resolution techniques for forming even finer patterns in a layer on a semiconductor device. As used herein, "direct-write lithography" refers to creating such patterns directly in the layer, without the intermediary of an imaging mask such as is used in conventional photolithography.